A process now used to prepare metal halide glasses having negligible OH absorption at 2.9.mu. fuses a mixture of ultra pure reagents such as zirconium tetrafluoride (ZrF.sub.4), barium fluoride (BaF.sub.2), and thorium fluoride (ThF.sub.4), in a platinum or gold crucible maintained at 900.degree. C. in an atmosphere of hydrogen fluoride (HF) and an inert gas. The hydrogen fluoride atmosphere is then replaced by a rectifying atmosphere of carbon tetrachloride (CCl.sub.4) and an inert gas and the melt is heated an additional 3 to 6 hours to remove the discoloration produced in the first stage of the process. In a subsequent stage the glass is molded, reheated, and cooled in the carbon tetrachloride and inert gas atmosphere.
In another process now used to make low OH content glasses, ammonium bifluoride (NH.sub.4 HF.sub.2) is mixed with the reagent metal fluorides. In this process, the bifluoride decomposes in the heating and fusing stage and generates an atmosphere of hydrogen fluoride. Discloration of the glass does not occur in this process and, consequently, subsequent rectification is not necessary.
The first described prior art process causes a dark discoloration of the glass melt during the HF processing stage. The removal of that discoloration requires subsequent processing to rectify. Another disadvantage of that process is that processing with CCl.sub.4 can lead to contamination and discoloration of the melt from crucible metal chlorides produced by the attack of nascent chlorine on the crucible material. If, for example, the crucible is of platinum, the nascent chloride tends to form platinum chloride. Further, where the reagent oxides and hydroxides in the melt are not entirely converted to fluorides in the HF treatment stage, the residual oxides and hydroxides may experience chlorination during CCl.sub.4 rectification. If that occurs, it introduces an additional component to the glass composition with subsequent loss in glass stability.
The process in which ammonium bifluoride is used to generate an atmosphere of HF has several disadvantages associated with adding ammonium bifluoride to the high purity reagents. As previously stated, an important use for fluoride glasses is for low loss optical fibers having minimum intrinsic optical attenuation in the 1 to 4.5.mu. region. That wavelength region is also the absorbance region for some transition and rare earth elements. For some of those impurities, substantial absorbance occurs even at the sub-ppb (sub- parts per billion) concentration level. The ammonium bifluoride now available from commercial sources is not of sufficient purity and its addition to the glass system introduces impurity contamination with a consequent increase in the optical loss of the glass.
In addition to impurity contamination another drawback arising from the use of ammonium bifluoride results from the nature of its decomposition products. At high temperatures, ammonium bifluoride decomposes into hydrogen fluoride and ammonium fluoride (NH.sub.4 F) as indicated by the following equation EQU NH.sub.4 HF.sub.2 .fwdarw.HF+NH.sub.4 F
The ammonium fluoride sublimes and condenses on the cooler surfaces within and surrounding the furnace with consequent contamination of the area. Processing of fluoride glasses with ammonium bifluoride in confined or controlled environments such as gloveboxes and clean rooms, results in contamination of the environment by the ammonium fluoride condensate and creates a source of extrinsic scattering loss for glasses quenched in that contaminated environment.